A disk drive comprising a read head for optically reading information recorded on a disk is known as an optical disk/magneto-optical disk drives such as CD, DVD, and MD.
In such a disk drive, an outer rotor type brushless motor is employed as a so-called spindle motor for rotatably driving a disk. A turntable on which a disk is placed is provided on a rotating shaft of the spindle motor so as to rotate integrally therewith, and the rotating shaft is rotatably supported by a bearing on the stator.
A sintered oil-impregnated metal bearing is used; this is because such a bearing is less expensive than a roller bearing and thus can reduce the costs of a motor and device. However, unlike a roller bearing, clearance is required between a metal bearing and rotating shaft, and due to this clearance axial run-out of the rotating shaft becomes larger than that with a roller bearing.
This axial run-out becomes disk surface run-out or disk axial run-out when a disk is rotatably driven, and is the cause of the drawback of a read head becoming unable to accurately read information from a disk.
Various constitutions and methods have been proposed as means for preventing axial run-out of a rotating shaft as above. In particular, the present applicant has filed applications, as in Laid-open Japanese Patent No. 2004-7905, wherein a constant force is exerted on a rotor not only in the rotating shaft direction (the thrust direction) but also in a radial direction using a magnetic force to cause the rotor to be slightly inclined and eccentric in a prescribed direction.
By thus applying a force is applied to a rotor in a prescribed direction, and causing the spindle motor to rotate in a state where the rotor is eccentric in a prescribed direction, axial run-out of a rotating shaft is inhibited and surface run-out and axial run-out of a disk is improved.
On the other hand, in the relative movement of a read head and disk in a disk drive, it is relatively easy for the head follow to disk run-out in the focus direction, and thus signals can be read even with a certain degree of disk run-out. However, in the tracking direction, even though it is possible for the head to follow the disk run-out, greater precision in terms of surface run-out and axial run-out is demanded when compared to the focus direction.
For this reason, a technology is disclosed in Laid-open Japanese Patent No. H09-7905, wherein read head moving direction is correlated with the direction of eccentricity of a spindle motor rotor.
As shown in Japanese Laid-open Patent No. 2004-7905, for a disk drive using an optical read head, a constitution wherein a rotor is eccentric in a head moving direction is considered effective. In such a constitution a prescribed section of a magnetic plate shaped stator core comprising a plurality of stacked magnetic plates is cut away, thereby creating magnetic unbalance with respect to a drive magnet provided on a rotor, thus causing the rotor to be eccentric in a certain direction.
Such a constitution requires not only the processing of each magnetic plate into a prescribed form, but also the assembly of the processed magnetic plates in a set combination. Further, because assembling a stator core as a drive device requires that the assembling direction be determined, such assembly is extremely troublesome.
Further, because a specialized motor is required for a disk drive, the use of such motor is also limited.